1. Field of the Invention
The technical scope of the present invention is that of hydrocarbon transport pipes able to regulate the temperature of the hydrocarbons.
2. Description of Related Art
To prepare for the depletion of subsea oil fields at low and medium temperature, which is to say at a temperature of less than 120° C., oil producers are obliged to exploit the deep geological layers of the subsea oil reserves, for example at depths of around 4000 or 6000 meters, or even deeper. Because of a mean geothermal gradient of 30° C./km, the hydrocarbons leaving the oil wells can reach temperatures of over 150° C. The temperatures observed can be, for example 210° C. or even 240° C. at the wellhead.
One technical problem is thus to provide installations adapted to these temperatures, both to support the thermal expansion by resisting to the stresses or by authorizing and controlling them, and to select materials adapted to these temperatures. Mechanical structures such as expansion loops, for example, enable substantial expansion, and mitigate the phenomenon of buckling of hydrocarbon pipelines. Another solution is disclosed in patent application GB-2188394 which describes a helical structure enabling the expansion of the pipelines. These approaches are not economically viable in the case of pipelines of several kilometers or several tens of kilometers in length and for which free expansion can reach several meters per kilometer.
In addition to the constraints depending on the dimensional variations, the constitutive metallic material of the pipeline has a maximum yield strength that corresponds to an acceptable maximum stress and that reduces as the temperature increases. The acceptable maximum stress is reduced, for example, by 75 MPa for carbon steel exposed to 200° C. and by 140 MPa for duplex steel, which is appreciable given a maximum elasticity of around 400 or 450 MPa at ambient temperature.
In the event of completely restrained thermal expansion, the stresses due to temperature changes increase by more than 2 MPa/° C. and it has been observed that high temperatures can lead to substantial increases in the thickness of the pipe walls to allow them to withstand the internal pressures. Restraining the thermal expansion thus results in additional force exerted on the pipeline.
There is, therefore, a strong need to reduce the temperature of hydrocarbon transport pipelines. This need for cooling is all the more important at the wellhead where the temperatures are the highest.
Even downstream of the well outlet, after part of the heat has dissipated, high temperatures can remain and cause technical problems. Chemical products injected into the wellstream namely have maximal operating temperatures. Thus, an anti-corrosion product is inoperative at above 100° C., for example. Electronic components may also be damaged or destroyed at temperatures above their maximum operating temperature.
Lowering the temperature thus enables the installations to operate more efficiently from a mechanical point of view but also enables chemical additives or electronic equipment to be used the closest from the well outlet.
A solution that is sometimes implemented consists in installing a portion of non-insulated piping at the well outlet to facilitate cooling. The length of such piping rarely exceeds a few tens of meters, that only enables marginal cooling of 10 to 20° C. when the oil well is in maximum production. Moreover, such a solution is not satisfactory for production stoppages involving a risk of forming plugs of hydrocarbons in the non-insulated part. Indeed, the temperature of the effluent cannot be allowed decrease to the temperature of the sea environment, typically 4° C., since this could lead to the formation of gas hydrates and other solids which could block the hydrocarbon pipeline.
Another issue is the variation of pressure and temperature at the well head as the oil reservoir is being exploited. Initially, the wellhead will be exposed to high temperature and pressure. This will require substantial cooling at the beginning of exploitation and less cooling, or even no cooling at all, at the end of exploitation.
There is thus a need for the temperature to be regulated in the hydrocarbon transport pipeline.
A subsea installation must furthermore meet the requirements of simplicity and robustness.